This invention relates to an AC solenoid apparatus for operating valves, such as hydraulic/air valves, or other machines. More particularly it relates to an improved AC solenoid of the so-called armature-in-tube type including an armature for moving a valve spool. The armature is immersed in oil to enable movement freely in the axial direction and further, has a tubular pressure proof part which forms an oil-tight seal to the outside. Prior art AC solenoids of the armature-in-tube type are shown in U.S. Pat. No. 3,633,139. In such prior art AC solenoids, since the armature is kept immersed in oil, there is need to seal off this portion with a pressure proof tube. This pressure proof tube has to be constructed, at least partially, of a nonmagnetic material because of the necessity for providing a magnetic path. In most cases, the whole tube is made from nonmagnetic material. In such a case the magnetic reluctance increases and the solenoid performance is significantly debased, resulting in the generation of only a weak operating force.
In the solenoid disclosed in U.S. Pat. No. 3,633,139, an exciting assembly comprising a coil and yoke provides the magnetic path. The pressure proof tube, which is contacted on its outer circumferential surface by the yoke, is subjected to a thermal treatment which places it in a magnetic state at those portions where it contacts the yoke and in a magnetic condition between those portions. Alternatively, a non-magnetic element may be welded at the intermediate portion.
However, such thermal treatment is complicated and there is a possibility that an undesirable effect will be produced around the boundary face between the magnetic and nonmagnetic materials and, when the welding technique is used, the thickness of the tube must be increased. The thermal treatment causes eddy currents to increase and both methods are expensive because, for example, it is necessary to grind the inner faces of both thermally-treated and welded tubes. Also, since no slot can be provided in the portion made from magnetic material, eddy currents are generated. The prior art solenoids have the disadvantage mentioned above.